Escherichia coli strain K12 (together with its plasmids and bacteriophage) has for decades been the most extensively studied bacterium. The genetic and biochemical information available for this strain probably surpasses that for any other living cell. With the advent of rapid and powerful techniques for sequencing and cloning DNA, the desirable goal of analyzing, in detail, physical, structural and organizational characteristics for the complete genome has become a realistic possibility. A set of overlapping lambda phage clones that systematically covers the entire chromosome is nearly finished and a detailed restriction map of the E. coli genome is being determined. The clones are also being assigned to their positions on the genetic map. The gene encyclopedia and restriction maps will be used to study global characteristics of the genome. The map will provide the most accurate to date determinations of the length of the chromosome and the distribution of sites will provide evidence for duplications, inversions and areas of unusual composition. Correlation with the genetic map will be used to quantify spaces between known genes and large incongruities may identify recombinational hot or cold spots. The encyclopedia will also be used to study gene expression by mapping entire sets of co-regulated genes such as all those responding to a particular environmental stimulus. An encyclopedia for the related bacterium, Serratia marcescens, will be constructed and organizational characteristics of the genomes compared.